Optical ring network with hub node and method

ABSTRACT

An optical network includes an optical ring and a plurality of add/drop nodes coupled to the optical ring. Each of the add/drop nodes is operable to passively add and drop one or more traffic streams to and from the optical ring, and each traffic stream comprises a channel. A hub node also coupled to the optical ring is operable to selectively pass and terminate individual traffic streams.

RELATED APPLICATIONS

This is a continuation-in-part application claiming the priority benefitof U.S. patent application Ser. No. 10/158,523 filed May 29, 2002entitled “Optical Ring Network with Optical Subnets and Method,” whichis hereby incorporated by reference.

TECHNICAL FIELD OF THE INVENTION

The present invention relates generally to optical transport systems,and more particularly to an optical ring network with hub node andmethod.

BACKGROUND OF THE INVENTION

Telecommunications systems, cable television systems and datacommunication networks use optical networks to rapidly convey largeamounts of information between remote points. In an optical network,information is conveyed in the form of optical signals through opticalfibers. Optical fibers comprise thin strands of glass capable oftransmitting the signals over long distances with very low loss.

Optical networks often employ wavelength division multiplexing (WDM) ordense wavelength division multiplexing (DWDM) to increase transmissioncapacity. In WDM and DWDM networks, a number of optical channels arecarried in each fiber at disparate wavelengths. Network capacity isbased on the number of wavelengths, or channels, in each fiber and thebandwidth, or size of the channels.

The typology in which WDM and DWDM networks are built plays a key rolein determining the extent to which such networks are utilized. Ringtopologies are common in today's networks. WDM add/drop units serve asnetwork elements on the periphery of such optical rings. By using WDMadd/drop equipment that each network element (node), the entirecomposite signal can be fully demultiplexed into its constituentchannels and switched (added/dropped or passed through).

SUMMARY OF THE INVENTION

The present invention provides an optical ring network with hub node andmethod. In one embodiment, an optical network includes an optical ringand a plurality of add/drop nodes coupled to the optical ring. Each ofthe add/drop nodes is operable to passively add and drop one or moretraffic streams to and from the optical ring, and each traffic streamcomprises a channel. A hub node also coupled to the optical ring isoperable to selectively pass and terminate individual traffic streams.

Technical advantages of the present invention include providing animproved optical ring network. In a particular embodiment, an opticalring comprising passive add/drop nodes is coupled to a hub node,providing for a network with relatively low cost and high capacity.

Another technical advantage of the present invention includes providinga high capacity, passive ring network. In a particular embodiment, byallowing terminable traffic streams within working paths of protectabletraffic, the overall capacity of the network may be increased throughwavelength reuse.

Another technical advantage of the present invention provides finegranularity between metro access and metro core environments dependingon customers' demand. The optical network of the present invention maybe easily upgraded by adding additional hub nodes, such that the each ofthe plurality of hub nodes may comprise gateway nodes between subnets,thus allowing for additional increases in network capacity at arelatively low cost.

It will be understood that the various embodiments of the presentinvention may include some, all, or none of the enumerated technicaladvantages. In addition, other technical advantages of the presentinvention may be readily apparent to one skilled in the art from thefollowing figures, description and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention and itsadvantages, reference is now made to the following description taken inconjunction with the accompanying drawings, wherein like numeralsrepresent like parts, in which:

FIG. 1 is a block diagram illustrating an optical ring network inaccordance with one embodiment of the present invention;

FIG. 2 is a block diagram illustrating details of an add/drop node ofFIG. 1 in accordance with one embodiment of the present invention;

FIG. 3 is a block diagram illustrating details of an optical coupler ofthe node of FIG. 2 in accordance with one embodiment of the presentinvention;

FIG. 4A is a block diagram illustrating details an optical wavelengthreuse gateway of FIG. 1 in accordance with one embodiment of the presentinvention;

FIG. 4B is a block diagram illustrating a mux/demux unit for the gatewayof FIG. 4A in accordance with another embodiment of the presentinvention;

FIG. 4C is a block diagram illustrating a mux/demux unit for the gatewayof FIG. 4A in accordance with yet another embodiment of the presentinvention;

FIG. 5 is a block diagram illustrating light paths of intra-subnetoptical signals of the optical network of FIG. 1 in accordance with oneembodiment of the present invention;

FIG. 6 is a block diagram illustrating protection switching and lightpath protection of the working light path of FIG. 5 in accordance withone embodiment of the present invention;

FIG. 7 is a block diagram illustrating a light path of an inter-subnetoptical signal of the optical network of FIG. 1 in accordance withanother embodiment of the present invention;

FIG. 8 is a block diagram illustrating protection switching and lightpath protection of the light path of FIG. 7 in accordance with oneembodiment of the present invention;

FIG. 9 is a flow diagram illustrating a method for transmitting trafficin an optical ring network with optical subnets in accordance with oneembodiment of the present invention;

FIG. 10 is a flow diagram illustrating a method for protection switchingin an optical ring network with optical subnets in accordance with oneembodiment of the present invention;

FIG. 11 is a block diagram illustrating an optical ring network inaccordance with another embodiment of the present invention;

FIG. 12A is a block diagram illustrating a combining element of anadd/drop node of FIG. 11 in accordance with one embodiment of thepresent invention;

FIG. 12B is a block diagram illustrating a combining element of anadd/drop node of FIG. 11 in accordance with another embodiment of thepresent invention;

FIG. 12C is a block diagram illustrating the combining element of FIG.12A with additional provisioning for transponder redundancy;

FIG. 13A is a block diagram illustrating a distributing element of anadd/drop node of FIG. 11 in accordance with one embodiment of thepresent invention;

FIG. 13B is a block diagram illustrating a distributing element of anadd/drop node of FIG. 11 in accordance with another embodiment of thepresent invention;

FIG. 14A illustrates details of an add/drop node of the network of FIG.11 in accordance with another embodiment of the present invention;

FIG. 14B illustrates details of a gateway node of the network of FIG. 11in accordance with another embodiment of the present invention;

FIG. 15 is a block diagram illustrating light paths of optical signalsof the optical network of FIG. 11 in accordance with one embodiment ofthe present invention;

FIG. 16 is a block diagram illustrating protection switching and lightpath protection of a traffic stream of FIG. 15 in accordance with oneembodiment of the present invention;

FIG. 17 is a flow diagram illustrating a method for protection switchingin an optical ring network in accordance with one embodiment of thepresent invention;

FIG. 18 is a block diagram illustrating an optical network in accordancewith yet another embodiment of the present invention;

FIG. 19 is a block diagram illustrating light paths of optical signalsof the optical network of FIG. 18 in accordance with one embodiment ofthe present invention;

FIG. 20 is a block diagram illustrating protection switching of theoptical network of FIG. 19 in accordance with one embodiment of thepresent invention;

FIG. 21 is a block diagram illustrating light paths of optical signalsof the optical network of FIG. 18 in accordance with another embodimentof the present invention;

FIG. 22 is a block diagram illustrating protection switching of theoptical network of FIG. 21 in accordance with one embodiment of thepresent invention;

FIG. 23 is a flow diagram illustrating a method for protection switchingin a hubbed passive optical ring network in accordance with oneembodiment of the present invention;

FIG. 24 is a block diagram illustrating a multi-subnet optical ringnetwork in accordance with another embodiment of the present invention;

FIG. 25 is a block diagram illustrating an add drop node of the networkof FIG. 24 in accordance with one embodiment of the present invention;

FIG. 26 is a block diagram illustrating a gateway node of the network ofFIG. 24 in accordance with one embodiment of the present invention; and

FIG. 27 is a block diagram illustrating a gateway node of the network ofFIG. 24 in accordance with another embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a block diagram illustrating an optical network 10 inaccordance with one embodiment of the present invention. In accordancewith this embodiment, the network 10 is an optical ring. An optical ringmay include, as appropriate, a single, unidirectional fiber, a single,bi-directional fiber, or a plurality of uni- or bi-directional fibers.In the illustrated embodiment, the network 10 includes a pair ofunidirectional fibers, each transporting traffic in opposite directions,specifically a first fiber, or ring, 16 and a second fiber, or ring, 18.Rings 16 and 18 connect a plurality of add/drop nodes (ADNs) 12 andoptical wavelength reuse gateways 14. Network 10 is an optical networkin which a number of optical channels are carried over a common path atdisparate wavelengths. The network 10 may be an wavelength divisionmultiplexing (WDM), dense wavelength division multiplexing (DWDM), orother suitable multi-channel network. The network 10 may be used in ashort-haul metropolitan network, and long-haul inter-city network or anyother suitable network or combination of networks.

Referring to FIG. 1, optical information signals are transmitted indifferent directions on the rings 16 and 18 to provide fault tolerance.The optical signals have at least one characteristic modulated to encodeaudio, video, textual, real-time, non-real-time and/or other suitabledata. Modulation may be based on phase shift keying (PSK), intensitymodulation (IM) and other suitable methodologies.

In the illustrated embodiment, the first ring 16 is a clockwise ring inwhich traffic is transmitted in a clockwise direction. The second ring18 is a counterclockwise ring in which traffic is transmitted in acounterclockwise direction. The ADNs 12, one embodiment of which isfurther described in reference to FIG. 2, are each operable to passivelyadd and drop traffic to and from the rings 16 and 18. In particular,each ADN 12 receives traffic from local clients and adds that traffic tothe rings 16 and 18. At the same time, each ADN 12 receives traffic fromthe rings 16 and 18 and drops traffic destined for the local clients. Asused herein, the term “each” means every one of at least a subset of theidentified items. In adding and dropping traffic, the ADNs 12 maymultiplex data from clients for transmittal in the rings 16 and 18 andmay demultiplex channels of data from the rings 16 and 18 for clients.Traffic may be dropped by making the traffic available for transmissionto the local clients. Thus, traffic may be dropped and yet continue tocirculate on a ring. The ADNs 12 communicate the traffic on the rings 16and 18 regardless of the channel spacing of the traffic—thus providing“flexible” channel spacing in the ADNs 12. “Passively” in this contextmeans the adding or dropping of channels without power, electricity,and/or moving parts. An active device would thus use power, electricityor moving parts to perform work. In a particular embodiment of thepresent invention, traffic may be passively added to and/or dropped fromthe rings 16 and 18 by splitting/combining, which is withoutmultiplexing/demultiplexing, in the transport rings and/or separatingparts of a signal in the ring.

Rings 16 and 18 and the ADNs 12 are subdivided into subnets 20 and 22,with the gateways 14 forming the subnet boundaries. A subnet may bedefined as a subset of nodes on a ring whose wavelengths are notisolated from each other and which may comprise traffic streams fromnodes within the subnet, but whose wavelengths are isolated from trafficstreams from other nodes on the ring, except for a minority ofwavelengths (at least during normal operations) that transport trafficstreams that pass through, enter or exit the subnet in order to reachtheir destination nodes. The gateways may be operable to terminateingress traffic channels from a subnet that have reached theirdestination ADNs (including those that have or will reach theirdestination nodes in an opposite direction) and to forward ingresstraffic channels from a subnet that have not reached their destinationADNs. In one embodiment, the gateway nodes may comprise a demultiplexerto demultiplex the signal into constituent traffic channels, switches toselectively terminate traffic channels, and a multiplexer to multiplexthe remaining signal before exiting the gateway. Further detailsregarding the gateways 14 are described below in reference to FIG. 4A.

Each ring 16 and 18 is open at least one point for each channel. Theopening for each channel in the rings 16 and 18 may be a physicalopening, an open, crossed, or other non-closed switch, a filter, adeactivated transmission device or other obstruction operable tocompletely or effectively terminate, and thus remove channels from therings 16 and 18 at the terminal points such that interference of eachchannel with itself due to recirculation is prevented or minimized suchthat the channels may be received and decoded within normal operatinglimits. As described further below in reference to FIG. 6, the rings 16and 18 may, in response to a line cut or other interruption, beprovisioned to terminate in ADNs 12 adjacent to the interruption usingswitch elements in ADNs 12. Switch elements may comprise simple on-offswitches, 2×2 switches, optical cross connects, or other suitable switchelements.

In one embodiment, a portion of the channels are open at the boundariesof the subnets at both gateways 14. Within each subnet, traffic ispassively added to and passively dropped from the rings 16 and 18,channel spacing is flexible, and the nodes are free to transmit andreceive signals to and from nodes within the subnet. Such traffic may bereferred to as “intra-subnet traffic.” Another portion of thetraffic—“inter-subnet traffic”—may travel to and from nodes in the othersubnet, and the lightpaths of such traffic would be open at only one ofthe gateways. Such inter-subnet traffic traverses or travels within atleast part of two subnets.

Because an intra-subnet traffic stream utilizes its wavelength/channelonly within its subnet, the wavelength/channel used for intra-subnettraffic in one subnet is free to be used in the other subnet by anothertraffic stream. In this way, the present invention increase the overallcapacity of the network, while maintaining flexible channel spacingwithin individual subnets.

Furthermore, it is possible to protect a first traffic stream in achannel within in a first subnet by assigning a terminable status to asecond channel stream using the same channel in the second subnet, suchthat the second channel stream becomes a protection channel access (PCA)stream. Terminable signals are signals that are terminated to provideprotection to other signals. Protectable signals are signals for whichprotection is provided. In this way, in the event of a line cut or otherinterruption causing the first traffic stream to not reach all of itsdestination nodes, the second traffic stream may be terminated and agateway switch for that channel closed, thus allowing the first trafficstream to travel through the gateway and through the second subnet backto the destination nodes of the first subnet and avoiding theinterruption. After the interruption has been repaired, the network mayrevert to its pre-interruption state such that open gateway switches forthe channel again separate the network into two subnets for the channel.Details of such protection switching are described further in referenceto FIG. 6.

A protocol for assigning channels to traffic in the network may bedevised to allow for efficient and simple provisioning of the network.For example, protection-switchable traffic from ADNs in subnet 20 isconveyed in odd-numbered channels and non-protected, terminable trafficfrom ADNs in subnet 20 is conveyed in even numbered channels, whereasprotection-switchable traffic from ADNs in subnet 22 is conveyed ineven-numbered channels and non-protected, terminable traffic from ADNsin subnet 22 is conveyed in odd-numbered channels. In this way, aprotection-switchable traffic stream in one subnet will be assured aprotection path occupied only by terminable traffic in the other subnet.In one embodiment, the protection-switchable traffic may comprisehigher-priority traffic than the terminable traffic; however, it will beunderstood that other divisions of the traffic streams intoprotection-switchable and terminable portions may be suitable ordesirable in other embodiments.

Inside a subnet, the optical fiber or fibers act as a shared medium. Thegateway dividing two sectors breaks the spatial continuity between thetwo-shared mediums. For a given network the number of sectors neededdepends on the maximum capacity of each node. Though network traffic isdynamic in one embodiment, the number of transponder cards needed ateach node to provision lightpaths makes the upper bound on traffic anestimable quantity. Let Tr_(i) be the upper bound on traffic (inlightpaths) emanating from node ‘i’ and (ΣTr_(i))_(max) is thecumulative maximum traffic in the ring. Further if the total number ofwavelengths (assuming equal channel spacing) is λ_(max), then themaximum number of subnets is given as S_(max)=(ΣTr_(i))_(max)/λ_(max)+1.

Algorithm: Initialize j ← 1; t ← 1; sum ← 0 for i=1:N sum=sum+cap(N_(t))if sum>=λ_(max) subnet(j) = node(t...N_(i−1)) sum ← 0; j=j+1 t ← N_(i)elseif i=N & j>1 subnet(j) = node(t...N) end endConsecutive nodes that have a cumulative bandwidth requirementapproximately equal or less than the total available bandwidth (inlightpaths) are grouped together into one subnet. The last node of eachsubnet may be a gateway. Moreover for an arbitrary network the lastsubnet may not be as heavily loaded as the other subnets.

Two kinds of lightpath establishment deserves attention, intra-subnetlightpath establishment and inter-subnet lightpath establishment. Thewavelength assignment algorithm may maximize wavelength reuse. It alsomay assign wavelengths heuristically such that all intra-subnet (ingressand egress nodes in the same subnet) lightpaths are assigned the lowestavailable wavelength. On the other hand inter-subnet lightpaths (thosewhose ingress and egress nodes are on different subnets or differentrings for that matter) are assigned on the highest possible wavelengths.This way we have a static load balancing which also may reduce thenumber of net transponder card type required in the ring.

In one embodiment, each subnet has a wavelength channel capacitysubstantially equal to the optical network. Substantially equal in thiscontext in one embodiment may mean the subnet has eighty percent of itswavelengths isolated from the other subnets and available forintra-subnet traffic. In other embodiments, substantially equal may meanninety percent another suitable percentage.

The network may be divided into subnets based on bandwidth usage pernode. For example, a network may have N nodes, the maximum capacity (interms of bandwidth) of the network, and the typical capacity per node.Let k₁ be the bandwidth required for the i^(th) node such that the totalneeded bandwidth needed in the network is

$\sum\limits_{i = 0}^{n}\;{k_{i}.}$Bandwidth is distributed to each node, and the first subnet is builtwhen either the total bandwidth is exhausted completely or when thesubnet bandwidth is such that addition of the next node would create aexcess bandwidth issue. This process is repeated until each node isplaced in a possible subnet.

The net number of subnets may be generally proportional to the totalcumulative minimum bandwidth required by all nodes in the network. Theprocedure for setting up subnets may be heuristic as well as static. ForN nodes, if there are D number of subnets and if G is the totalbandwidth needed then G/D need not necessarily be N due to the excessbandwidth. In one embodiment of the present invention, up to sixteenpercent (16%), the total number of transporter cards can be saved whencompared to a standard network.

Each node may have a minimum fixed capacity for transmission. Each nodemay also have a maximum variable capacity for transmission and this isgenerally the upper bound on its maximum traffic requirement. Within thesubnet the nodes may be free to communicate with each other. Each nodeis allotted a band for transmission that can “listen” to the entirebandwidth for reception. This band is a dedicated band, and in additioncan also have a small overlap section which can be used fornon-dedicated applications by intelligent multiplexing of statisticalbandwidth access.

FIG. 2 is a block diagram illustrating details of an ADN 12 of FIG. 1 inaccordance with one embodiment of the present invention. Referring toFIG. 2, the node 12 comprises counterclockwise transport element 50,clockwise transport element 52, distributing element 80, managingelement 110, and combining element 130. In one embodiment, the elements50, 52, 80, 110, and 130 as well as components within the elements maybe interconnected with optical fiber links. In other embodiments, thecomponents may be implemented in part or otherwise with planar waveguidecircuits and/or free space optics. In addition, the elements of ADN 12may each be implemented as one or more discrete cards within a cardshelf of the ADN 12. Exemplary connectors 70 for a card shelf embodimentare illustrated by FIG. 2. The connectors 70 may allow efficient andcost effective replacement of failed components. It will be understoodthat additional, different and/or other connectors may be provided aspart of the ADN 12.

Transport elements 50 and 52 may each comprise passive couplers or othersuitable optical splitters/couplers 60, ring switch 62, amplifier 64,and OSC filters 66. Ring switch 62 may be a 2×2 switch or other switchelement operable to selectively open the connected ring 16 or 18. In the2×2 embodiment, the switch 62 includes a “cross” or open position and a“through” or closed position. The open position allows the ring openingsin the ADNs 12 to be selectively reconfigured to provide protectionswitching.

Amplifier 64 may comprise an EDFA or other suitable amplifier. In oneembodiment, the amplifier is a preamplifier and may be selectivelydeactivated to open a connected ring 16 or 18 to provide protectionswitching in the event of failure of the adjacent switch 62. In thisembodiment, the preamplifier 64 and the switch 62 are disposed in thetransport elements 50 and 52 inside of the OSC filters and between theingress filter 66 and the splitter/couplers 60. Thus, the OSC signal maybe recovered regardless of the position of switch 62 or operation ofpreamplifier 64. In another embodiment, OSC signals may be transmittedin-band with revenue-generating traffic by placing OSC filters betweenthe couplers 60. OSC filters 66 may comprise thin film type, fibergrating or other suitable type filters.

The transport segments may comprise either a single splitter/coupler ora plurality of couplers/splitters which allow for the passive adding anddropping of traffic. In the illustrated embodiment, counterclockwisetransport segment 50 in the illustrated embodiment includes a passiveoptical splitter set having a counterclockwise drop coupler 58 and acounterclockwise add coupler 72. The counterclockwise transport element50 further includes OSC filters 54 and 74 at the ingress and egressedges, and counterclockwise amplifier 56 between the ingress OSC filter54 and counterclockwise ring switch 63.

Clockwise transport segment 52 includes a passive optical splitter setincluding clockwise drop coupler 82 and clockwise add coupler 84.Clockwise transport element 52 further includes OSC filters 76 and 86,clockwise amplifier 78, and clockwise ring switch 65. OSC filters 76 and86 are disposed at the ingress and egress edges of the clockwisetransport element 52. The clockwise amplifier 78 is disposed between theingress OSC filter 76 and the clockwise ring switch 65.

Distributing element 80 may comprise a plurality of distributingamplifiers. In this embodiment, the distributing element 80 may comprisea drop coupler feeding into the distributing amplifiers which eachinclude an amplifier and an optical splitter. For example, a firstdistributing amplifier may include amplifier 94 and optical splitter 95while a second distributing amplifier may include amplifier 96 andsplitter 97. The amplifiers 94 and 96 may comprise EDFAs or othersuitable amplifiers. Splitters 95 and 97 may comprise splitters with oneoptical fiber ingress lead and a plurality of optical fiber drop leads98. The drop leads 98 may be connected to one or more filters 100 whichin turn may be connected to one or more drop optical receivers 102.

Combining element 130 may be a amplified combiner and may comprise asplitter 136 with a plurality of optical fiber add leads 138 which maybe connected to one or more add optical senders 140 associated with aclient. Splitter 136 further comprises two optical fiber egress leadswhich feed into amplifiers 132 and 134. Amplifiers 132 and 134 maycomprise EDFAs or other suitable amplifiers.

Managing element 110 may comprise OSC senders 116 and 122, OSCinterfaces 114 and 120, OSC receivers 112 and 118, and an elementmanagement system (EMS) 124. Each OSC sender, OSC interface and OSCreceiver set forms an OSC unit for one of the rings 16 and 18 in thenode 12. The OSC units receive and transmit OSC signals for the EMS 124.The EMS 124 may be communicably connected to a network management system(NMS) 126. NMS 126 may reside within node 12, in a different node, orexternal to all of the nodes 12.

EMS 124 and/or NMS 126 may comprise logic encoded in media forperforming network and/or node monitoring, failure detection, protectionswitching and loop back or localized testing functionality of thenetwork 10. Logic may comprise software encoded in a disk or othercomputer-readable medium and/or instructions encoded in an applicationspecific integrated circuit (ASIC), field programmable gate array(FPGA), or other processor or hardware. It will be understood thatfunctionality of EMS 124 and/or NMS 126 may be performed by othercomponents of the network 200 and/or be otherwise distributed orcentralized. For example, operation of NMS 126 may be distributed to theEMS of nodes 12 and 14 and the NMS 126 thus omitted as a separate,discrete element. Similarly, the OSC units may communicate directly withNMS 126 and EMS 124 omitted.

The ADN 12 further comprises counterclockwise add fiber segment 144,counterclockwise drop fiber segment 146, clockwise add fiber segment142, clockwise drop fiber segment 148, OSC fiber segments 150, 152, 154,and 156, and connectors 68. As illustrated, connection 68 may be angledto avoid reflection. As previously described a plurality of passivephysical contact connectors 70 may be included where appropriate so asto communicably connect the various elements of ADN 12.

In operation, the transport elements 50 and 52 are operable to passivelyadd local traffic to the rings 16 and 18 and to passively drop at leastlocal traffic from the rings 16 and 18. The transport elements 50 and 52are further operable to passively add and drop the OSC signal to andfrom the rings 16 and 18. More specifically, in the counterclockwisedirection, OSC filter 54 processes an ingress optical signal fromcounterclockwise ring 18. OSC filter 54 filters OSC signal from theoptical signal and forwards the OSC signal to the OSC interface 114 viafiber segment 150 and OSC receiver 112. OSC filter 54 also forwards orlets pass the remaining transport optical signal to amplifier 56. Byplacing the OSC filter 54 before ring switch 63, the ADN 12 is able torecover the OSC signal regardless of the position of the ring switch 63.

Amplifier 56 amplifies the signal and forwards the signal to ring switch63. Ring switch 63 is selectively operable to transmit the opticalsignal to coupler 58 when the ring switch 63 is set to the through(closed) setting, or to transmit the optical signal to an OSA connector68 when the ring switch 63 is set to the cross (open) setting. Furtherdetails regarding the OSA connectors are described below.

If ring switch 63 is set in the cross position, the optical signal isnot transmitted to couplers 58 and 72, the ring 18 is open at the ADN12, and dropping of traffic from the ring 18 at node 12 and pass-throughof traffic does not occur at node 12. If the ring switch 63 is set inthe through position, the optical signal is forwarded to couplers 58 and72 and adding and dropping of traffic to and from the ring 18 at node 12may occur at node 12.

Coupler 58 passively splits the signal from switch 63 into two generallyidentical signals. A passthrough signal is forwarded to coupler 72 whilea drop signal is forwarded to distributing element 80 via segment 146.The signals may be substantially identical in content, although powerand/or energy levels may differ. Coupler 72 passively combines thepassthrough signal from coupler 58 and an add signal comprising localadd traffic from combining element 130 via fiber segment 144. Thecombined signal is passed to OSC filter 74.

OSC filter 74 adds an OSC signal from the OSC interface 114, via the OSCsender 116 and fiber segment 152, to the combined optical signal andforward the combined signal as an egress transport signal to ring 18.The added OSC signal may be locally generated data or may be receivedOSC data passed through by the EMS 124.

In the clockwise direction, OSC filter 76 receives an ingress opticalsignal from clockwise ring 16. OSC filter 76 filters the OSC signal fromthe optical signal and forwards the OSC signal to the OSC interface 120via fiber segment 154 and OSC receiver 118. OSC filter 76 also forwardsthe remaining transport optical signal to amplifier 78.

Amplifier 78 amplifies the signal and forwards the signal to ring switch65. Ring switch 65 is selectively operable to transmit the opticalsignal to coupler 82 when the ring switch 65 is set to the throughsetting, or to transmit the optical signal to an OSA connector 68 whenthe ring switch 65 is set to the cross setting.

If the ring switch 65 is set in the cross position, the optical signalis not transmitted to couplers 82 and 84, the ring 16 is open at thenode 12, and dropping of traffic the ring 16 and “pass-through” oftraffic does not occur at node 12. If the ring switch 65 is set in thethrough position, the optical signal is forwarded to couplers 82 and 84and adding and dropping of traffic to and from the ring 16 may occur atnode 12.

Coupler 82 passively splits the signal from switch 65 into generallyidentical signals. A passthrough signal is forwarded to coupler 84 whilea drop signal is forwarded to distributing unit 80 via segment 148. Thesignals may be substantially identical in content and/or energy. Coupler84 passively combines the passthrough signal from coupler 82 and an addsignal comprising local add traffic from combining element 130 via fibersegment 142. The combined signal is passed to OSC filter 86.

OSC filter 86 adds an OSC signal from the OSC interface 120, via the OSCsender 122 and fiber segment 156, to the combined optical signal andforwards the combined signal as an egress transport signal to ring 16.As previously described, the OSC signal may be locally generated data ordata passed through by EMS 124.

Prior to addition to the rings 16 and 18, locally-derived traffic istransmitted by a plurality of add optical senders 140 to combiningelement 130 of the node 12 where the signals are combined, amplified,and forwarded to the transport elements 50 and 52, as described above,via counterclockwise add segment 144 and clockwise add segment 142. Thelocally derived signals may be combined by the optical coupler 136, by amultiplexer or other suitable device.

Locally-destined traffic is dropped to distributing element 80 fromcounterclockwise drop segment 146 and clockwise drop segment 148.Distributing element 80 splits the drop signal comprising thelocally-destined traffic into multiple generally identical signals andforwards each signal to an optical receiver via a drop lead 98. Thesignal received by optical receivers 102 may first be filtered byfilters 100. Filters 100 may be tunable filters or other suitablefilters and receivers 102 may be broadband or other suitable receivers.

EMS 124 monitors and/or controls all elements in the node 12. Inparticular, EMS 124 receives an OSC signal in an electrical format viaOSC filters 66, OSC receivers 112 and 118, OSC senders 116 and 122, andOSC interfaces 114 and 120. EMS 124 may process the signal, forward thesignal and/or loop back the signal. Thus, for example, the EMS 124 isoperable to receive the electrical signal and resend the OSC signal tothe next node, adding, if appropriate, node-specific error informationor other suitable information to the OSC.

In one embodiment each element in a node 12 monitors itself andgenerates an alarm signal to the EMS 124 when a failure or other problemoccurs. For example, EMS 124 in node 12 may receive one or more ofvarious kinds of alarms from the elements and components in the node 12:an amplifier loss-of-light (LOL) alarm, an amplifier equipment alarm, anoptical receiver equipment alarm, optical sender equipment alarm, adistributing amplifier LOL alarm, a distributing amplifier equipmentalarm, an amplified combiner LOL alarm, an amplified combiner equipmentalarm, or other alarms. Some failures may produce multiple alarms. Forexample, a fiber cut may produce amplifier LOL alarms at adjacent nodesand also error alarms from the optical receivers.

In addition, the EMS 124 may monitor the wavelength and/or power of theoptical signal within the node 12 via connections (not shown) betweenconnectors 68 and an optical spectrum analyzer (OSA) communicablyconnected to EMS 124.

The NMS 126 collects error information from all of the nodes 12 and 14and is operable to analyze the alarms and determine the type and/orlocation of a failure. Based on the failure type and/or location, theNMS 126 determines needed protection switching actions for the network10. The protection switch actions may be carried out by NMS 126 byissuing instructions to the EMS in the nodes 12 and 14.

Error messages may indicate equipment failures that may be rectified byreplacing the failed equipment. For example, a failure of one of theamplifiers in the distributing element may trigger a distributingamplifier alarm. The failed amplifier can then be replaced. A failedcoupler in the distributing element may be likewise detected andreplaced. Similarly, a failure of an optical receiver or sender maytrigger an optical receiver equipment alarm or an optical senderequipment alarm, respectively, and the optical receiver or senderreplaced as necessary.

In another embodiment of the present invention, redundant ring switchesmay be provided in the transport elements. The redundant ring switchesmay allow for continued circuit protection in the event of switchfailure, and failed ring switches may be replaced without interferingthe node operations or configuration. Ring switch failure may comprise,among other things, failure of a ring switch to change from the crossposition to a through position, failure of a ring switch to change froma through position to the cross position, or the switch becoming fixedin an intermediate position. The redundant ring switches may thus allowfor protection switching in the event that a switch fails to switch fromthe closed position to the open position. Alternatively, redundancy inthe event of a switch stuck in the closed position can be accomplishedwithout a redundant switch by turning off the amplifier for that ring inthe node with the failed switch, thus effectively terminating the signalat the amplifier.

In various other embodiments of the ADNs 12, the ADNs 12 may compriseactive nodes, passive nodes, or a combination of active and passivenodes. Nodes may be passive in that they include no switches, switchableamplifiers, or other active devices. Nodes may be active in that theyinclude optical switches, switchable amplifiers, or other active devicesin the transport elements or otherwise in the node. Passive nodes may beof a simpler and less expensive design. In one embodiment, the networkcomprises a combination of active and passive nodes. In this way, activenodes may provide for protection switching functionality while theaddition of passive nodes may allow for additional ADNs in the networkwhile minimizing the additional cost associated with the additionalnodes.

In other embodiments of the present invention, described in more detailin reference to FIGS. 11–16, the distributing element and the combiningelement may comprise a divided distributing element (DDE) and a dividedcombining element (DCE), respectively. Whereas in the embodiment shownin FIG. 2 the combining element forwards traffic to both ringssimultaneously and each receiver of the distributing element receivestraffic from both rings, in the DDE/DCE embodiments, individual trafficchannels may be forwarded to the clockwise ring or to thecounterclockwise ring by the DCE, and received by the DDE from theclockwise ring or from the counterclockwise ring. During protectionswitching, the DCE may switch from forwarding a particular channel fromone ring to the other. In this way, the DDE/DCE embodiments provide foreither the two-subnet configuration shown in FIG. 1 or a configurationwith a greater number of subnets.

FIG. 3 is a block diagram illustrating details of an opticalsplitter/coupler 60 of the node of FIG. 2 in accordance with oneembodiment of the present invention. In this embodiment, the opticalsplitter/coupler 60 is a fiber coupler with two inputs and two outputs.The optical splitter/coupler 60 may in other embodiments be combined inwhole or part with a waveguide circuit and/or free space optics. It willbe understood that the splitter/coupler 60 may include one or any numberof any suitable inputs and outputs and that the splitter/coupler 60 maycomprise a greater number of inputs than outputs or a greater number ofoutputs than inputs.

Referring to FIG. 3, the optical splitter/coupler 60 comprises a mainbody 180, first entry segment 182, second entry segment 184, first exitsegment 186, and second exit segment 188 First entry segment 182 andfirst exit segment 186 comprise a first continuous optical fiber. Secondentry segment 184 and second exit segment 188 comprise a secondcontinuous optical fiber. Outside of the main body 180, segments 182,184, 186, and 188 may comprise a jacket, a cladding, and a core fiber.Inside the main body 180, the jacket and cladding may be removed and thecore fibers twisted or otherwise coupled together to allow the transferof optical signals and/or energy of the signals between and among thefirst and second continuous optical fibers. In this way, the opticalsplitter/coupler 60 passively combines optical signals arriving fromentry segments 182 and 184 and passively splits and forwards thecombined signal via exit segments 186 and 188. A plurality of signalsmay be combined and the combined signal split by combining andthereafter splitting the combined signal or by simultaneously combiningand splitting the signals by transferring energy between fibers.

The optical splitter/coupler 60 provides flexible channel-spacing withno restrictions concerning channel-spacing in the main streamline. In aparticular embodiment, the coupler has a directivity of over −55 dB.Wavelength dependence on the insertion loss is less than about 0.5 dBover a 100 nm range. The insertion loss for a 50/50 coupler is less thanabout −3.5 dB.

FIG. 4A is a block diagram illustrating details an optical wavelengthreuse gateway of the network of FIG. 1 in accordance with one embodimentof the present invention. In this embodiment, each channel (wavelength)is separated from the multiplexed signal and independently passed orterminated. In other embodiments, groups of channels may be passed orterminated. As previously described, the gateway is disposed between,and may form the boundary of, neighboring subnets. A channel reusegateway in one embodiment may be any suitable node, nodes or element ofone or more nodes that is configurable to selectively isolate or exposewavelengths between nodes in one or more directions of a ring or othersuitable network configuration. Wavelength reuse may in one embodimentbe the use of a wavelength in a ring or other suitable network totransport disparate traffic streams in a same fiber or direction.

Referring to FIG. 4A, the wavelength reuse gateway comprises amanagement element 110 comprising OSC senders 116 and 122, OSCinterfaces 114 and 120, OSC receivers 112 and 118, and an EMS 124, asdescribed above in reference to FIG. 2. The EMS 110 is connected totransport elements 200 and 202 via OSC fiber segments 150, 152, 154, and156, again as described in reference to FIG. 2.

As described above in reference to FIG. 2, counterclockwise transportelement 200 comprises OSC filters 54 and 74, pre-amplifier 56, andpost-amplifier 78. Clockwise transport element 202 comprises OSC filters76 and 86, pre-amplifier 56, and post-amplifier 78. Transport elements200 and 202 further comprises mux/demux units 214. Mux/demux units 214may each comprise demultiplexer 206, multiplexer 204, and switchelements which may comprise an array of switches 210 or other componentsoperable to selectively pass or terminate a traffic channel. In aparticular embodiment, multiplexers 204 and demultiplexers 206 maycomprise arrayed waveguides. In another embodiment, the multiplexers 204and the demultiplexers 206 may comprise fiber Bragg gratings. Theswitches 210 may comprise 2×2 or other suitable switches, opticalcross-connects, or other suitable switches operable to terminate thedemultiplexed traffic channels.

Pre-amplifiers 56 may use an automatic level control (ALC) function withwide input dynamic-range and automatic gain control (AGC).Post-amplifiers 78 may deploy AGC to realize gain-flatness against inputpower variation due to channel add/drop, too. In a particularembodiment, the amplifiers 56 and 78 may be gain variable amplifiers,such as, for example, as described in U.S. Pat. No. 6,055,092.

In operation, counterclockwise transport element 200 receives a WDMsignal, comprising a plurality of channels, from ring 18. OSC filter 54filters the OSC signal from the optical signal as described above andthe remaining optical signal is forwarded to amplifier 56, as describedabove. Demultiplexer 206 demultiplexes the optical signal into itsconstituent channels. Switches 210 selectively forward or terminatechannels to multiplexer 204. Multiplexer 204 multiplexes the channelsinto one optical signal and to forward the optical signal to OSC filter74. OSC filter 74 adds the OSC signal from EMS 110, and the ring 18receives the egress signal.

Clockwise transport segment 202 receives an optical signal from ring 16.OSC filter 76 filters the OSC signal from the optical signal asdescribed above and the remaining optical signal is forwarded toamplifier 78, as described above. Demultiplexer 206 demultiplexes theoptical signal into its constituent channels. Switches 210 selectivelyforward or terminate channels to multiplexer 204. Multiplexer 204multiplexes the channels into one optical signal and to forward theoptical signal to OSC filter 86. OSC filter 86 adds the OSC signal fromEMS 110, and the ring 18 receives the egress signal.

EMS 110 configures mux/demux units 214 to provide protection switching.Protection switching protocols are described in greater detail below. Inaccordance with various embodiments, gateways 14 may be further operableto add and drop traffic from and to local clients and/or to and fromother networks.

In accordance with various other embodiments, gateway 14 may be furtherprovisioned to passively add and drop traffic to the optical rings. Forexample, in accordance with one embodiment, transport units 50 and 52 ofFIG. 2 may be added to gateway 14 on the rings 16 and 18 next to themux/demux units 214. In another embodiment, traffic may be added via theadd and drop leads of 2×2 switches within the mux/demux units. Furtherdetails regarding this latter embodiment are described below inreference to FIG. 4B.

FIG. 4B is a block diagram illustrating a mux/demux unit of the gatewayof FIG. 4A in accordance with another embodiment of the presentinvention. In accordance with this embodiment, mux/demux unit 240 ofFIG. 4B may be substituted for mux/demux modules 214 of FIG. 4A.

Referring to FIG. 4B, mux/demux unit 240 comprises demultiplexer 206 andmultiplexer 204 as described above in reference to FIG. 4A. In place ofthe plurality of switches 210 are a plurality of 2×2 switch/attenuatorsets each comprising 2×2 switch 241, variable optical attenuator (VOA)242, optical splitter 243, photodetector 245, and controller 244. VOA242 attenuates the ingress signal to a specified power level based on afeedback loop including splitter 243 which taps the signal,photodetector 245 which detects the power level of the signal andfeedback controller 244 which controls VOA 244 based on the detectedpower level. In this way, the rings may be opened for a particularchannel by switching the 2×2 switch to the “cross” position, and thepower level of the “through” signal when the 2×2 switch is in the“through” position may be adjusted. Also, as described above, trafficmay be added and/or dropped from the rings via the add and drop leads of2×2 switches 241.

FIG. 4C is a block diagram illustrating a mux/demux unit of the gatewayof FIG. 4A in accordance with yet another embodiment of the presentinvention. In accordance with this embodiment, the unit is anoptical-electrical-optical (O-E-O) unit. Unit 246 of FIG. 4C may besubstituted for mux/demux modules 214 of FIG. 4A.

Referring to FIG. 4C, O-E-O unit 246 comprises demultiplexer 206 andmultiplexer 204 as described above in reference to FIG. 4A. In place ofthe plurality of switches 210 are a plurality of O-E-O elements, eachcomprising receivers 247, switches 248, and transmitters 249. Ademultiplexed signal is passed to the receiver 247 corresponding to itschannel, wherein the optical signal is converted to an electricalsignal. Switches 248 are operable to selectively pass or terminate theelectrical signal from receiver 247. A signal passed through via switch248 is forwarded to transmitter 249, wherein the signal is converted toan optical signal. Optical signals from the plurality of transmitters249 are multiplexed in multiplexer 204 and the multiplexed signalforwarded as described above in reference to FIG. 4A. Thus, O-E-O unit246 may act as a regenerator of the signals passing through the gateway14.

FIG. 5 is a block diagram illustrating light paths of optical signals ofthe optical network of FIG. 1 in accordance with one embodiment of thepresent invention. In FIG. 5, paths of exemplary intra-subnet signalsare illustrated. For ease of reference, only high-level details of thetransport elements of ADNs 12 and gateways 14 are shown. In addition,ADNs 12 are assigned individual reference numbers, with ADNs 252, 254,and 256 within subnet 20 and ADNs 260, 262, and 264 within subnet 22.Gateways 14, forming the boundary between subnets 20 and 22 are alsoassigned individual reference numbers 250 and 258.

Lightpaths 266 and 268 represent a traffic stream added to the networkfrom an origination node ADN 262 (the “ADN 262 traffic stream”) in thecounterclockwise and clockwise directions, respectively. In theillustrated embodiment, the intended destination node of the ADN 262traffic stream is ADN 264. Lightpath 266 terminates at gateway 258 at anopen switch (or “cross” state of 2×2 switch) in counterclockwisetransport segment 200 corresponding to the channel of the trafficstream. Lightpath 268 terminates at gateway 250 in clockwise transportsegment 202 at an open switch in clockwise transport segment 202corresponding to the channel of the traffic stream. It will be notedthat, although FIG. 5 shows node 264 as the destination node, thetraffic also reaches the drop ports of ADN 260 and of gateways 250 and258 (if any). Likewise, traffic originating from nodes 252, while shownas having a destination node ADN 256, also reaches the drop ports of ADN254 and of gateways 250 and 258 (if any). Thus, the network has abroadcasting function within the subnets.

In the illustrated embodiment, lightpaths 270 and 272 represent atraffic stream added to the network from an origination node ADN 252(the “ADN 252 traffic stream”) in the counterclockwise and clockwisedirections, respectively. In the illustrated embodiment, the intendeddestination node of the ADN 252 traffic stream is ADN 256. Lightpath 270terminates at gateway 25 at an open switch in counterclockwise transportsegment 200 corresponding to the channel of the traffic stream.Lightpath 272 terminates at gateway 258 at an open switch in clockwisetransport segment 202 corresponding to the channel of the trafficstream.

The ADN 262 traffic stream and the ADN 252 traffic stream may representdifferent traffic but may be conveyed within the same channel, orwavelength. However, since the ADN 262 traffic stream and the ADN 252traffic stream are isolated within different subnets. In this way, theoverall capacity of the network is increased for that channel, eventhough channel flexibility is maintained within each subnet.

Either the ADN 262 traffic stream or the ADN 252 traffic stream (eachusing the same channel) may be assigned a terminable status.“Terminable” in this context means that that stream may be selectivelyterminated to provide a protection path for the another stream. Theother stream may be a protectable stream, “protectable” meaning that itmay be protected in the event of an interruption of one of the lightpaths of that traffic stream via protection switching. The light path ofthe protectable traffic stream may be termed the “working path” and thelight path of the terminable traffic stream may be termed the“protection path.” Thus, in the illustrated example, a client addingtraffic to the network via ADN 262 may pay a premium for a working paththat will be protected in the event of a line cut or other interruption.Such traffic may comprise voice, video, or other real-time ortime-sensitive traffic. The client adding traffic to the network at ADN252 may pay a lesser amount to use the protection path of the premiumclient of the other subnet, subject to termination if necessary toprotect the working path. An example of such protection switching isshown in FIG. 6.

FIG. 6 is a block diagram illustrating protection switching and lightpath protection of the working light path of FIG. 5 in accordance withone embodiment of the present invention. In the example shown in FIG. 6,as described above, the path 268 of the ADN 262 traffic stream fromorigination node 262 to destination node 264 is dedicated as the workingpath, whereas the light paths 270 and 272 of the ADN 252 traffic streamare a protection paths. The ADN 252 traffic stream and the ADN 262traffic stream in the illustrated embodiment are carried on the samechannel.

In the illustrated example, the line cut 274 prevents the ADN 262traffic stream as shown in FIG. 5 from reaching its destination node264. Specifically, the line cut prevents traffic from travelling on linepath 268 to ADN 264. Pursuant to the protection switching protocol, theADN 252 traffic stream is terminated, and the switches 210 in gateways258 and 250 corresponding to the wavelength of the ADN 252 trafficstream and the ADN 262 traffic stream are closed, allowing the ADN 262traffic stream to pass through gateway 258 and enter subnet 20 and becarried in a counterclockwise direction to ADN 264. In this way, each ofthe destination nodes of the ADN 262 traffic stream receive the ADN 262traffic stream. In order to ensure an opening in the rings 16 and 18 inthe channel of the ADN 262 traffic stream during protection switching,switches 62 in the transport element 50 of ADN 262 and switch 62 in thetransport element 52 of ADN 264 are opened. In this way, channelinterference is prevented, for example, if the line cut 274 only affectsone ring, or during repair operations. In a particular embodiment, forany working channel in a working path interruption, the correspondingprotection channel in the protection path is terminated and the switchesin the gateways are opened. If work channels are not affected, thesystem continues as before.

After repair of the line cut, the network is reverted to itspre-protection switching state shown in FIG. 5. Specifically, theswitches in gateways 258 and 250 corresponding to the wavelength of theADN 252 traffic stream and the ADN 262 traffic stream are opened, thusconfining the ADN 262 traffic stream to the subnet 22, and the switches62 in ADNs 262 and 264 are closed. In this way, the “protection path” isrecovered. The ADN 252 traffic stream may then be transmitted on paths270 and 272.

In a particular embodiment, the NMS of the network 10 may be operable tochoose the shortest protection path from among a plurality of possibleprotection paths.

FIG. 7 is a block diagram illustrating a light path of an optical signalof the optical network 10 of FIG. 1 in accordance with anotherembodiment of the present invention. In the embodiment shown in FIG. 7,paths of an exemplary intra-subnet signal is illustrated.

In the embodiment shown in FIG. 7, the ADN 262 traffic stream is aninter-subnet traffic stream carried on light paths 350 and 352, with atleast a portion of the light paths carried in both subnets 20 and 22. Inthe illustrated embodiment, the destination node of the ADN 262 trafficstream is ADN 254. The optical rings 16 and 18 are open for the channelof the ADN 262 traffic stream at switches 210 of gateway 250corresponding to that channel, but are closed at switches 210 of gateway258. Switches in the ADNs are in the closed, pass-through state.

FIG. 8 is a block diagram illustrating protection switching and lightpath protection of the working light path of FIG. 7 in accordance withone embodiment of the present invention. In the example shown in FIG. 7,as described above, the ADN 262 traffic stream is an intra-subnettraffic stream.

In the illustrated example, the line cut 284 prevents the ADN 262traffic stream from reaching its destination node. Specifically, theline cut prevents traffic from travelling on line path 350 to ADNs 254,252, and 250.

Pursuant to the protection switching protocol, the switches 210 ingateway 250 corresponding to the wavelength of the ADN 262 trafficstream is closed, allowing the ADN 262 traffic stream to pass throughgateway 250 and be carried in a clockwise direction to ADNs 254 and 252.In this way, the destination node 254 of the ADN 262 traffic streamreceives the ADN 262 traffic stream.

In order to ensure an opening in the rings 16 and 18 in the channel ofthe ADN 262 traffic stream during protection switching, switches 62 inthe transport element 50 of ADN 254 and switch 62 in the transportelement 52 of ADN 256 are opened. In this way, channel interference isprevented, for example, if the line cut 274 only affects one ring, orduring repair operations.

After repair of the line cut, the network is reverted to itspre-protection switching state shown in FIG. 7. Specifically, the switchin gateway 258 corresponding to the wavelength of the ADN 262 trafficstream is opened and the switches 62 in ADNs 254 and 256 are closed.

FIG. 9 is a flow diagram illustrating a method for transmitting trafficin an optical network in accordance with one embodiment of the presentinvention.

Referring to FIG. 9, the method for transmitting traffic in an opticalnetwork begins with step 400, wherein traffic is passively added anddropped from an optical ring at each of a plurality of ADNs andtransported in the ring in a specific wavelength, or channel.

Proceeding to step 402, those traffic channels that have reached theirdestination ADN are terminated at a plurality of discrete points alongthe ring. In one embodiment, such termination occurs at the switches ofone or more gateways 14, such that the gateways 14 formed the boundariesof subnets within the network. For intra-subnet traffic streams, thesource and destination ADNs all are within a subnet. For inter-subnettraffic streams, the source and destination ADNs may be within two ormore subnets.

Proceeding to step 404, traffic channels that have not reached all theirdestination add/drop nodes are forwarded through the gateways to allowthe destination node to be reached. It will be understood that thegateway may be reconfigured to pass and terminate certain specifiedwavelengths and thus not dynamically whether a traffic stream has or hasnot reach its destination. Such forwarding may occur in the ordinarycourse for inter-subnet traffic. In addition, as described in referenceto FIG. 6, gateways may forward intra-subnet traffic so as to protectthat traffic in the event of a line cut or other interruption.

At step 406, it is ensured that no channel interference is occurring. Ina particular embodiment, this may be accomplished by confirming that thegateways are configured to not pass through channels which wouldinterfere with the intended network traffic.

FIG. 10 is a flow diagram illustrating a method for protection switchingfor the optical network of FIG. 1 in accordance with one embodiment ofthe present invention. As described above in reference to FIG. 1, thenetwork comprising a first optical ring and a second optical ring and aplurality of subnets and the traffic carried within a signal comprisinga wavelength.

Referring to FIG. 10, the method begins with step 450 wherein an linecut or other interruption in the working path of a high-priority trafficstream is detected. An interruption may be any event causing one or morechannels to not reach their destination over a working or existing path.The detection may be via a loss-of-light alarm received at the EMS 124of an ADN adjacent to the interruption. The EMS 124 may process theerror message and forward the message to NMS 126, which may process thecommands necessary to complete the remainder of the method and transmitthose commands to the EMSs 124 of the ADNs 12 and the gateways 14.

Proceeding to step 452, the interruption is isolated. In a particularembodiment, the NMS 126 directs the EMS 124 of the ADN 12 downstream ofthe interruption in the clockwise direction to open the switch 62 inclockwise transport element 52 and the switch 62 in counterclockwiseelement 50. Thus, in the example shown in FIG. 6, the switches 62 in theADNs 262 and 264 are opened as shown in response to the line cut 274.Opening the switches 62 at these adjacent locations may prevent channelinterference, for example, if the line cut 274 only affects one ring, orduring repair operations.

Proceeding to step 454, terminable traffic is corresponding to theworking paths which are to be protected, terminated along the protectionpath. Terminable traffic may remain if the corresponding working path isnot interfered with by a fiber cut or other interruption. In aparticular embodiment, the NMS 126 directs the EMS 124 of any ADNs 12 inanother subnet transmitting traffic in the same channel as thehigh-priority traffic stream to cease adding traffic to the network.

Proceeding to step 456, the gateways are reconfigured to allow theprotected traffic to proceed along the protection path. In a particularembodiment, this may be accomplished by closing the previously-openedswitches 210 corresponding to that wavelength in the gateway or gateways14 along the protection path. The gateways may be otherwise suitablyreconfigured by mechanical, electrical, optical or other means toestablish protection paths between subnets. The gateway and the othernodes and elements may be controlled locally or remotely by logic orotherwise.

At step 458, the protected traffic is forwarded along the protectionpath to its destination node or nodes. At decisional step 460, it isdetermined whether the interruption has been repaired. If not, themethod returns to step 458 and the protection path continues to carrythe protected traffic. If the interruption has been repaired, the methodproceeds to step 462 wherein the network is reverted to itspre-interruption state, such reversion comprising closing the openedswitches and again adding the terminable traffic to its channel in thenetwork. Upon reversion the method may repeat in response to at leastdetection of another interruption.

FIG. 11 is a block diagram illustrating an optical network in accordancewith another embodiment of the present invention. Specifically, FIG. 11represents an embodiment the present invention operable for a networkwith three subnets, instead of the two subnets of FIG. 1. It will beunderstood that the present invention, in the particular embodimentshown in FIGS. 11–17, may be utilized in networks with two, three, ormore subnets.

Referring to FIG. 11, the network 500 includes a first fiber optic ring510 and a second fiber optic ring 512 connecting a plurality of add/dropnodes (ADNs) 508 and optical wavelength reuse gateways 514. As with thenetwork 10 of FIG. 1, network 500 is an optical network in which anumber of optical channels are carried over a common path at disparatewavelengths, may be an wavelength division multiplexing (WDM), densewavelength division multiplexing (DWDM), or other suitable multi-channelnetwork, and may be used in a short-haul metropolitan network, andlong-haul inter-city network or any other suitable network orcombination of networks.

In network 500, also as in network 10 of FIG. 1, optical informationsignals are transmitted in different directions on the rings 510 and 512to provide fault tolerance. The optical signals have at least onecharacteristic modulated to encode audio, video, textual, real-time,non-real-time and/or other suitable data. Modulation may be based onphase shift keying (PSK), intensity modulation (IM) and other suitablemethodologies.

In the illustrated embodiment, the first ring 510 is a clockwise ring inwhich traffic is transmitted in a clockwise direction. The second ring512 is a counterclockwise ring in which traffic is transmitted in acounterclockwise direction. The ADNs 508 are similar to the ADNs 12 ofFIG. 2 in that each are operable to add and drop traffic to and from therings 510 and 512 and comprise transport elements 50 and 52, and amanaging element 110. However, in one embodiment, in place of combiningelement 130 in ADN 508 is a divided combining element (DCE). A DCE,described in further detail and in various embodiments in FIGS. 12A–12B,may be provisioned to forward a first specified subset of the totalchannels originating from the ADN 508 to first ring 510 and a secondspecified subset of the total channels to the second ring 512. Switchesin the DCE may allow for a particular traffic stream to be selectivelyforwarded to a different ring during protection switching. Also, in oneembodiment, in place of distributing element 80 in ADNs 508 is a divideddistributing element (DDE). A DDE, as described in further detail and invarious embodiments in FIGS. 13A–13B, may be provisioned to receivetraffic from ring 510 in a first subset of receivers, and traffic fromring 512 in a second subset of receivers. Whereas in the embodimentshown in FIG. 2 the combining element forwards traffic to both ringssimultaneously and each receiver of the distributing element receivestraffic from both rings, in the DDE/DCE embodiments, individual trafficchannels may be forwarded to the clockwise ring or to thecounterclockwise ring by the DCE, and received by the DDE from theclockwise ring or from the counterclockwise ring. During protectionswitching, the DCE may switch from forwarding a particular channel fromone ring to the other. In this way, the DDE/DCE equipped ADNs 508 allowfor three or more protection-switchable subnets.

In one embodiment, network 500 may carry 40 channels, with theodd-numbered channels comprising channels λ₁, λ₃, λ₅, λ₇, etc., throughλ₃₉ and the even numbered channels comprising channels λ₂, λ₄, λ₆, λ₈,etc., through λ₄₀. In accordance with this embodiment, the DCE may beprovisioned to, during normal operations, forward higher prioritytraffic in odd-numbered channels to clockwise ring 510 and ineven-numbered channels to counterclockwise ring 512. Lower-priority,terminable traffic may be forwarded by the DCE in even-numbered channelsto clockwise ring 510 and in odd-numbered channels to counterclockwisering 512. In the event of a line cut or other interruption, and asdescribed further below in reference to FIGS. 15 and 16, the DCE mayswitch interrupted high priority traffic to the other direction on theother ring. Wavelength assignment may be based on the shortest path fromorigination node to destination node.

Similar to the ADN 12 of FIG. 2, each ADN 508 receives traffic from therings 510 and 512 and drops traffic destined for the local clients. Inadding and dropping traffic, the ADNs 508 may multiplex data fromclients for transmittal in the rings 510 and 512 and may demultiplexchannels of data from the rings 510 and 512 for clients. Traffic may bedropped by making the traffic available for transmission to the localclients. Thus, traffic may be dropped and yet continue to circulate on aring. Again, similar to the ADN 12 of FIG. 2, the transport elements ofthe ADNs 508 communicate the received traffic on the rings 510 and 512regardless of the channel spacing of the traffic—thus providing“flexible” channel spacing in the ADNs 508.

Rings 510 and 512 and the ADNs 508 are subdivided into subnets 502, 504,and 506, with the gateways 514 forming the subnet boundaries. Thegateways may comprise gateways 14 of FIG. 4A or other suitable gateways.During protection switching, as described in further detail below inreference to FIGS. 15 and 16, the gateways 514 may be reconfigured toallow protected traffic to pass through.

As described with the network 10 of FIG. 1, each ring 510 and 512 isopen at least one point for each channel, and the rings 510 and 512 may,in response to a line cut or other interruption, be provisioned toterminate in ADNs 12 adjacent to the interruption using 2×2 switches inADNs 12. As with network 10, network 500 may comprise both intra-subnettraffic and inter-subnet traffic.

In accordance with the embodiments shown in FIGS. 11–16, it may bepossible to increase the capacity of a network by up to twice the numberof gateways in the network. For example, a three-subnet network asillustrated in FIG. 11 with three gateways may have a capacity of up tosix times the capacity of a network without such a subnet configuration.A four-subnet network with four gateways may have a capacity of up toeight times the capacity of a network without such a subnetconfiguration.

In accordance with another embodiment of the present invention, node 12of FIG. 2 may be further modified such that 2×2 switch 63 may be placedbetween drop coupler 58 and add coupler 72, and 2×2 switch 65 may beplaced between drop coupler 82 and add coupler 84. In this way, in theevent of protection switching which opens switches 62, traffic may stillreach the drop couplers 58 and 82.

FIG. 12A is a block diagram illustrating a divided combining element(DCE) of an add/drop node of the network of FIG. 11 in accordance withone embodiment of the present invention. In the embodiments shown inFIGS. 12A and 12B, the DCE comprises two separate or separable combiningelements, each of which receive traffic from a different fiber ordirection.

Referring to FIG. 12A, DCE 550 comprises a clockwise amplified combiner552 and a counterclockwise amplified combiner 554. Clockwise amplifiedcombiner 552 comprises amplifier 132, as described above in reference toFIG. 2, and splitter 556 with a plurality of optical fiber add leads562. Counterclockwise amplified combiner 554 comprises amplifier 134, asdescribed above in reference to FIG. 2, and splitter 558 with aplurality of optical fiber add leads 564.

Optical senders 140, described above in reference to FIG. 2, may beassociated with a local client and are each coupled to one of aplurality of switches 560. Switches 560 are operable to forward trafficto either clockwise amplified combiner 552 or to counterclockwiseamplified combiner 554. Each traffic stream may be associated with adedicated transmitter. Because traffic streams may be directed to one oftwo ring directions, two different traffic streams may, in oneembodiment, be transmitted on the same wavelength but in differentdirections.

In operation, an optical signal may be transmitted from optical sender140 to switch 560, forwarded by switch 560 to one of combiner 552 orcombiner 554, combined with other signals, amplified, and forwarded toclockwise ring 510 via lead 142 or to counterclockwise ring 512 via lead144.

For purposes of protection switching, optical signals may be eitherterminated at optical sender 140 or the direction of the optical signalchanged via switch 560. Further details regarding protection switchingis described in reference to FIGS. 14–16.

FIG. 12B is a block diagram illustrating a DCE 600 of an add/drop nodeof the network of FIG. 11 in accordance with another embodiment of thepresent invention. In the embodiment shown in FIG. 12B, in contrast toDCE 550 of FIG. 12A which is provisioned to forward a given channel onlyin one ring direction, DCE 600 of FIG. 12B may be provisioned to eithera) forward all channels from optical senders 140 in both directions, orb) to forward a given channel only in one ring direction. This dualfunctionality enables DCE 600 to be used either as a component in an ADNthat is part of a two-subnet network as described in reference to FIGS.1–8, or as a component of an ADN that is part of a three (or greaternumber) subnets network as described in reference to FIGS. 11–16.

Referring to FIG. 12B, DCE 600 comprises amplifiers 132 and 134,splitters 556 and 558, and add leads 562 and 564 as described above inreference to FIG. 12A. Switches 610 and 612 may be set in a firstposition in which signals from all optical senders 140 are transmittedto both clockwise and counterclockwise rings 510 and 512 via splitter604; or switches 610 and 612 may be set in a second position in whichsignals from splitter 556 are only forwarded to clockwise ring 510 vialead 142 and signals from splitter 556 are only forwarded tocounterclockwise ring 512 via lead 144. When set in the first position,DCE 600 functions in a generally equivalent manner as combining element130 of FIG. 2. When set in the second position, DCE 600 functions in agenerally equivalent manner as DCE 550 of FIG. 12A. Note that switches610 and 612 are not used for protection switching in the networkdescribed in reference to FIGS. 11–16; instead the DCE 600 isprovisioned in the second position and remains in the second positionwhether during normal or protection operations.

Splitters 606 and 608 may ensure that an optical signal has anequivalent coupler loss whether the switches 610 and 612 are set in thefirst position or the second position.

In the embodiment illustrated in FIG. 12B, two-position switch 560 ofDCE 550 has been replaced with three-position switch 602. Three-positionswitch 602, which may be used in either of the illustrated DCEembodiments, allows for a signal from optical sender 140 to be sent toone of two amplified combiners or to be terminated at switch 602. Inthis way, a signal can be terminated (for example, if necessary duringprotection switching) without shutting off the optical sender 140.

FIG. 12C is a block diagram illustrating the DCE 550 of FIG. 12Aprovisioned with features providing for transmitter transponderredundancy. Referring to FIG. 12C, working traffic destined forclockwise amplified combiner 552 may travel via lead 620 to transponder624 and to leads 562 via switch 616, provisioned as shown. A protectionchannel may be transmitted via lead 622, switch 618, transponder 626,and switch 614 to leads 564. In the event of a failure of transponder624, switches 614, 616, and 618 would be switched from their illustratedpositions to the alternative position such that traffic from lead 620may reach lead 562 via transponder 626.

FIG. 13A is a block diagram illustrating a divided distributing element(DDE) of an add/drop node of the network of FIG. 11 in accordance withone embodiment of the present invention. In the embodiments shown inFIGS. 13A and 13B, the DDEs comprises two separate or separabledistributing elements, each of which forward traffic to a differentfiber or direction.

Referring to FIG. 13A, DDE 650 comprises a clockwise amplifieddistributor 652 and a counterclockwise amplified distributor 654.Clockwise amplified distributor 652 comprises amplifier 94, as describedabove in reference to FIG. 2, and splitter 656 with a plurality ofoptical fiber drop leads 662. Counterclockwise amplified distributor 654comprises amplifier 96, as described above in reference to FIG. 2, andsplitter 658 with a plurality of optical fiber drop leads 664.

Optical filters 100 and receivers 102, described above in reference toFIG. 2, may be associated with a local client and are each coupled toone of a plurality of switches 660. Switches 660 are operable to forwardtraffic from either clockwise amplified distributor 652 or fromcounterclockwise amplified distributor 654. Each traffic stream may beassociated with a dedicated receiver.

In operation, an optical signal may be dropped from the transportelements 50 or 52 and forwarded to distributors 652 or 654 via dropleads 148 or 146, respectively. The signal is amplified and split bysplitters 656 or 658 and forwarded by a switch 660 to an optical filter100. Optical filter 100 selectively passes a channel to a receiver 102.

For purposes of protection switching, switch 660 is operable such that agiven receiver at a destination node during normal operations thatreceives an optical signal from a first ring may, during protectionswitching, receive that signal from the second ring. Further detailsregarding protection switching is described in reference to FIGS. 14–16.

FIG. 13B is a block diagram illustrating a DDE of an add/drop node ofthe network of FIG. 11 in accordance with another embodiment of thepresent invention. In the embodiment shown in FIG. 13B, DDE 600 of FIG.13B may be provisioned to either a) forward all channels from the ringsto each of the filters 100, or b) to forward channels from only one ringdirection. This dual functionality enables DCE 600 to be used either asa component in an ADN that is part of a two-subnet network as describedin reference to FIGS. 1–8, or as a component of an ADN that is part of athree (or greater number) subnets network as described in reference toFIGS. 11–17. The dual functionality also provides for protectionswitching such that a given receiver may receive a traffic stream from afirst ring direction during normal operations, and may receive thetraffic from the second ring direction during switching operations.

Referring to FIG. 13B, DDE 700 comprises amplifiers 94 and 96, splitters656 ad 658, and drop leads 662 and 664 as described above in referenceto FIG. 13A. In the illustrated embodiment, drop leads 662 and 664 areconnected directly to filters 100. Splitters 704 and 706 may ensure thatan optical signal has an equivalent coupler loss whether the switches608 and 610 are set in the first position or the second position. Duringnormal switching operations at a destination node, switches 608 and 610may be set in a first position in which signals from both clockwise andcounterclockwise rings 510 and 512 are sent to each receiver via coupler702. During protection switched operations, switches 608 and 610 may beset in a second position in which signals from clockwise ring 510 vialead 148 are only dropped to drop leads 662 via splitter 656, andsignals from counterclockwise ring 512 via lead 146 are only dropped todrop leads 664 via splitter 658. Thus, a given receiver may receive atraffic stream from a first ring direction during normal operations, andmay receive the traffic from the second ring direction during switchingoperations. Alternatively, DDE 700 may be provisioned to operate in agenerally equivalent manner as distributing element 80 of FIG. 2 andsuitable for a two-subnetwork network by setting DDE 700 in the firstposition for both normal and protection operations.

FIG. 14A illustrates details of an add/drop node of the network of FIG.11 in accordance with another embodiment of the present invention. Inthe embodiment of FIG. 14, OSC signals are transmitted in-band withrevenue-generating traffic. The node 712 may in another embodiment, beprovisioned for external OSC signals (as illustrated in FIG. 2).

Referring to FIG. 14A, the node 712 comprises DCE 550 and DDE 650 asdescribed above in reference to FIGS. 12A and 13A, respectively.Counterclockwise transport element 714 comprises amplifier 56, switch63, counterclockwise drop coupler 58, and counterclockwise add coupler72 as described above in reference to counterclockwise transport segmentof FIG. 2. However, in place of OSC filter 54 and 74, a single OSCrejection filter 716 is provisioned between couplers 58 and 72.

Likewise clockwise transport element 716 comprises clockwise dropcoupler 82, clockwise add coupler 84, amplifier 78, and switch 65 asdescribed in reference to clockwise transport segment 52 of FIG. 2.However, in place of OSC filter 76 and 86, a single OSC rejection filter718 is provisioned between couplers 82 and 84.

In this embodiment, in operation, OSC signals are transmitted in-band.OSC receiver 112 is operable to receive the OSC signal from theclockwise ring 510 via one of the drop leads 662 and OSC receiver 122 isoperable to receive the OSC signal from the counterclockwise ring 512via one of the leads 664. Filters 722 and 724 are operable toselectively filter the OSC data from the optical signals distributed bythe DDE 650. OSC units 114 and 120 transmit the OSC data to EMS 124 tobe processed by NMS 126 as described above in reference to FIG. 2. OSCsenders 116 and 118 are operable to transmit the clockwise andcounterclockwise signals to the DCE 550, respectively, via one of addleads 562 and one of add leads 564.

FIG. 14B illustrates details of a gateway node of FIG. 11 in accordancewith another embodiment of the present invention. Gateway 726 of FIG.14B comprises the elements of gateway 14 of FIG. 4A, but is furtherprovisioned to add and drop traffic from rings 510 and 512 via dropcouplers disposed at the ingress side of mux/demux units 214 and addcouplers disposed at the egress side of mux/demux units 214. The gatewaynode of FIG. 14B may be utilized in the embodiments described inreference to FIGS. 11–23 wherein the ADNs utilize DCEs and DDEs.

Referring to FIG. 14B, gateway 726 is provisioned to drop traffic fromrings 510 and 512 via drop leads 744 and 748, respectively. From each ofrings 510 and 512, traffic may be dropped via a drop coupler 728 to anamplified distributor comprising an amplifier 94, and a splitter 95.Likewise, in the illustrated embodiment, local traffic may be added toring 510 via lead 746 and to ring 512 via lead 742. Amplified combinersleading to leads 746 and 742 comprise combiners 735 and amplifiers 740,and are operable to combine and amplify the locally-derived traffic. Addcouplers 730 coupled to rings 510 and 512 are operable to add thetraffic from leads 746 and 742. In accordance with an alternativeembodiment, the add/drop functionality of gateway 14 may be omitted.

FIG. 15 is a block diagram illustrating light paths of optical signalsof the optical network of FIG. 11 in accordance with one embodiment ofthe present invention. In FIG. 15, for ease of reference, onlyhigh-level details of the transport elements of ADNs 508 and gateways514 are shown. In addition, ADNs 508 are assigned individual referencenumbers, with ADNs 516 and 518 within subnet 502, ADNs 520 and 522within subnet 504, and ADNs 524 and 526 within subnet 506. Gateways 514,forming the boundary between subnets 502, 504, and 506 are also assignedindividual reference numbers 528, 530 and 532.

In the illustrated embodiment, four traffic streams are shown. Trafficstream 750 is a counterclockwise stream originating from ADN 520 anddestined for ADN 508. Traffic stream 752 is a clockwise streamoriginating from ADN 520 and destined for ADN 522. Traffic stream 754 isa counterclockwise stream originating from ADN 526 and destined for ADN524. Traffic stream 756 is a clockwise stream originating from ADN 524and destined for ADN 526. Traffic streams 752 and 756 terminate atgateway 514 at an open switch in clockwise transport segment 202corresponding to the channel of the traffic streams. Traffic streams 750and 752 terminate at gateway 514 at the open switch in thecounterclockwise transport segment 200 corresponding to the channel ofthe traffic stream. Traffic streams 750, 752, 754, and 756 are carriedon the same channel or wavelength; however, the streams are transmittedfrom a separate optical sender within the DCEs of their respectiveorigination ADNs.

In the illustrated embodiment, during normal operations, protectabletraffic is forwarded in clockwise ring 510 in odd-numbered channels andin even-numbered channels to counterclockwise ring 512. Terminabletraffic may be forwarded in clockwise ring 510 in even-numbered channelsand in odd-numbered channels to counterclockwise ring 512. Each of thetraffic streams 750, 752, 754, and 756 is carried on the same,even-numbered channel (“Channel A”). Channel A may comprise λ₂ oranother even-numbered channel. Thus, traffic streams 750 and 754 are onworking paths and may represent higher-priority traffic streams forwhich a customer has paid a premium, and streams 752 and 756 mayrepresent lower-priority priority on protection paths for which acustomer has paid a lower cost. As shown in FIG. 16, streams 752 and 756may be interrupted during protection switching to protect ahigher-priority stream.

FIG. 16 is a block diagram illustrating protection switching and lightpath protection of the traffic stream 750 of FIG. 14 in accordance withone embodiment of the present invention.

In the event of a line cut or other interruption, an alternate lightpath is created for protectable channels that are prevented fromreaching all of their destination nodes due to the interruption. If thealternate line path would result in interference from traffic in thesame channel from other ADNs in other subnets, the DCE 550 in theinterfering ADN may terminate that traffic. As previously noted, it willbe understood that other divisions of traffic besides odd and even andother conventions may be utilized without departing from the scopepresent invention.

In the illustrated example, the line cut 560 prevents traffic stream 750from reaching all of its destination nodes in the path shown on FIG. 15.Pursuant to the protection switching protocol of this embodiment, first,traffic streams 752 and 756 are terminated. Then, the DCE of ADN 520switches traffic stream 750 from a counterclockwise to a clockwisedirection. Traffic streams 752 and 756 are terminated, and the 2×2switches in gateways 532 and 528 corresponding to Channel A are closedto allow Channel A to pass through. In this way, an alternate path forstream 750 from ADN 520 to ADN 516 is created with no interference fromother traffic streams on Channel A.

Depending upon the embodiment of the DCE (as shown in FIGS. 12A, 12B, orin another suitable embodiment), the termination of traffic streams 752and 756 may be by switching off the optical sender or by switching athree-position switch to a non-forwarding position.

In order to ensure an opening in the rings 510 and 512 during protectionswitching, switches 62 in the transport element 50 of ADN 516 and switch62 in the transport element 52 of ADN 518 are opened. In this way,channel interference is prevented, for example, if the line cut 560 onlyaffects one ring, or during repair operations.

After repair of the line cut, the network is reverted to itspre-protection switching state shown in FIG. 15. Specifically, theswitches in gateways 528 and 532 corresponding to Channel A are openedand the switches 62 in ADNs 516 and 518 are closed. Traffic stream 750is reverted to a counterclockwise direction, and traffic streams 752 and756 may restart.

FIG. 17 is a flow diagram illustrating a method for protection switchingfor the optical network of FIG. 11 in accordance with one embodiment ofthe present invention.

Referring to FIG. 17, the method begins with step 800 wherein a firstset of protectable traffic streams are forwarded in even channels in thecounterclockwise ring. Proceeding to step 802, a second set ofprotectable traffic streams are forwarded in odd channels on theclockwise ring. At step 804, a first set of terminable traffic isforwarded in even channels in the clockwise ring, and, at step 806, asecond set of terminable traffic is forwarded in odd channels in thecounterclockwise ring. In this way, each channel in each direction isoccupied by a traffic stream, thus efficiently utilizing networkcapacity. In a particular embodiment, the protectable traffic streamsare higher priority traffic streams for which a customer has paid apremium, whereas the terminable traffic streams are lower prioritytraffic streams.

At decisional step 808, it is determined whether has been aninterruption of a working path of a protectable traffic stream. Suchinterruption may comprise a line cut or other interruption that preventsthe protectable traffic stream from reaching all of its destinationnodes. If no interruption has occurred, the method returns to step 800.If an interruption has occurred, then, at step 810, the interruption isisolated. In a particular embodiment, the isolation of the interruptioncomprises opening the clockwise ring 510 at the add/drop node clockwiseof the interruption by opening the switch 62 in the clockwise transportelement, and opening the counterclockwise ring at the add/drop nodecounterclockwise of the interruption by opening the switch 62 in thecounterclockwise transport element.

Proceeding to step 812, any existing, terminable traffic is terminatedalong the protection path. At step 814, the gateways are reconfigured toallow the protected traffic to proceed along the protection path. In aparticular embodiment, this may be accomplished by closing thepreviously-opened switches 210 corresponding to that wavelength in thegateway or gateways 514 along the protection path.

Proceeding to step 816, a switch in the DCE of the origination ADN ofthe interrupted traffic switches the direction of the interruptedtraffic. At step 818, the interrupted traffic is transmitted in theprotection path. At decisional step 820, it is determined whether theinterruption has been repaired. If the interruption has not beenrepaired, the method returns to step 818 and the interrupted trafficcontinues to be transmitted in the protection path. If the interruptionhas been repaired, the method proceeds to step 822 wherein the networkis reverted to its pre-interruption state, and the method has reachedits end.

FIG. 18 is a block diagram illustrating another embodiment of an opticalnetwork. In the embodiment shown in FIG. 18, the network 900 comprises aplurality of add/drop nodes 902 and a hub node 904. Clockwise ring 901and counterclockwise ring 903 connect the nodes.

Add/drop nodes 902 may each comprise an add/drop node 12 as described inreference to FIG. 2 or another suitable add/drop node. Hub node 904 maycomprise a gateway node 14 as described in reference to FIG. 4A oranother suitable gateway node.

Switches within gateway node 904 may be open for specific wavelengthsduring normal operations, thus opening the network at the gateway nodeand preventing signal interference. The switches may also allow forprotection switching of a traffic stream in the event of a line cut orother interruption. In the event of such an interruption, the trafficstream may be able to reach its destination node by travelling along theopposite direction as during normal operations and passing through apreviously open switch which is closed for protection switching,allowing the traffic stream to pass through the gateway and to reach thedestination node.

In another embodiment, the add/drop nodes 902 may comprise DDEs andDCEs, allowing for separation of traffic received from and forwarded tothe clockwise and counterclockwise rings. In this embodiment, suchseparation allows for a given wavelength to be used as a working path inone direction and a protection channel access (PCA) in the otherdirection and thus may increase overall network capacity by up to twotimes, while still providing for protection switching functionality.

Network 900 may be particular suitable for connection to existinglong-haul or metro-core networks, which may be provisioned to easilyconnect through a hub node. In addition, 900 may be easily upgradable toa network with 2, 3 or more subnets in accordance with the embodimentspreviously described.

FIG. 19 is a block diagram illustrating light paths of optical signalsof the optical network of FIG. 18 in accordance with one embodiment ofthe present invention. For ease of reference, only high-level details ofthe transport elements of ADNs 902 and hub node 904 are shown. In theembodiment shown in FIG. 19, ADNs 902 comprise DCE 600 and DDE 700, theswitches in each of DCE 600 and DDE 700 provisioned to transmit the sametraffic in both the clockwise and counterclockwise directions.Alternatively, ADNs 902 may, in the embodiment shown in FIG. 19,comprise combining element 130 and distributing element 80 of FIG. 2, orother suitable combining or distributing elements.

Referring to FIG. 19, lightpaths 910 and 912 represent a traffic streamadded to the network from an origination node ADN 906 (the “ADN 906traffic stream”) in the counterclockwise and clockwise directions,respectively. In the embodiment shown in FIG. 19, the intendeddestination node of the ADN 906 traffic stream is ADN 908. Lightpath 922terminates at hub node 904 at an open switch (or “cross” state of 2×2switch) in clockwise transport segment 202 corresponding to the channelof the traffic stream. Lightpath 920 also terminates at hub node 904 incounterclockwise transport segment 202 at an open switch incounterclockwise transport segment 200 corresponding to the channel ofthe traffic stream. It will be noted that, although FIG. 19 shows node908 as the destination node, the traffic also reaches the drop ports ofADNs 910, 912, 914, 916, and 918. Thus, the network has a broadcastingfunction.

FIG. 20 is a block diagram illustrating protection switching and lightpath protection of the light path of FIG. 19 in accordance with oneembodiment of the present invention. Line cut 924 prevents the ADN 906traffic stream as shown in FIG. 19 from reaching its destination node908. Pursuant to the protection switching protocol, switches 210 in hubnode 904 corresponding to the wavelength of the ADN 906 traffic streamare closed, allowing the ADN 906 traffic stream to pass through hub node904. In this way, destination node 908 of the ADN 906 traffic streamreceives the ADN 906.

In order to ensure an opening in the rings 901 and 903 in the channel ofthe ADN 906 traffic stream during protection switching, switch 62 in thetransport element 50 of ADN 906 and switch 62 in the transport element52 of ADN 908 may be opened. In this way, channel interference isprevented, for example, if the line cut 924 only affects one ring, orduring repair operations. After repair of the line cut, the network isreverted to its pre-protection switching state shown in FIG. 19.

FIG. 21 is a block diagram illustrating light paths of optical signalsof the optical network of FIG. 18 in accordance with another embodimentof the present invention. In the embodiment shown in FIG. 21, ADNs 902comprise DCE 600 and DDE 700, the switches in each of DEC 600 and DDE700 provisioned to transmit a first traffic stream in the clockwisedirection and a second traffic stream in the counterclockwisedirections. Alternatively, ADNs 902 may, in the embodiment shown in FIG.19, comprise DCE 550 of FIG. 12A and DDE 650 of FIG. 13A, or othersuitable divided combining or divided distributing elements.

Referring to FIG. 21, lightpath 928 represents the first, or clockwise,traffic stream added to the network from origination node ADN 906 withADN 908 as its destination node. Lightpath 926 represents a second, orcounterclockwise, traffic stream added to the network from originationnode 906 with ADN 910 as its destination node As described above inreference to FIGS. 11–17, first lightpath 928 may comprise a workinglightpath which may comprise a protectable traffic stream. Secondlightpath 926 may comprise a protection channel access (PCA) lightpathcomprising, during normal operations, a terminable traffic stream.Lightpath 928 terminates at hub node 904 at an open switch (or “cross”state of 2×2 switch) in clockwise transport segment 202 corresponding tothe channel of the traffic stream. Lightpath 926 also terminates at hubnode 904 in counterclockwise transport segment 202 at an open switch incounterclockwise transport segment 200 corresponding to the channel ofthe traffic stream.

FIG. 22 is a block diagram illustrating protection switching and lightpath protection of the working path of FIG. 21 in accordance with oneembodiment of the present invention. Line cut 930 prevents trafficstream 928 as shown in FIG. 21 from reaching its destination node 908.Pursuant to the protection switching protocol, traffic corresponding tolight path 926 is terminated. Switches 210 in hub node 904 correspondingto the wavelength of the ADN 928 traffic stream are closed, allowing the928 traffic stream to pass through hub node 904. In this way,destination node 908 continues to receive the 928 traffic stream.

In order to ensure an opening in the rings 901 and 903, switch 62 in thetransport element 50 of ADN 906 and switch 62 in the transport element52 of ADN 908 are opened. In this way, channel interference isprevented, for example, if the line cut 930 only affects one ring, orduring repair operations. After repair of the line cut, the network isreverted to its pre-protection switching state shown in FIG. 21.

FIG. 23 is a flow diagram illustrating a method for protection switchingfor the optical network of FIG. 10 in accordance with one embodiment ofthe present invention.

Referring to FIG. 23, the method begins with step 1000 wherein a firstset of protectable traffic streams are forwarded in even channels in thecounterclockwise ring. Proceeding to step 1002, a second set ofprotectable traffic streams are forwarded in odd channels on theclockwise ring. At step 1004, a first set of terminable traffic isforwarded in even channels in the clockwise ring, and, at step 1006, asecond set of terminable traffic is forwarded in odd channels in thecounterclockwise ring. In this way, each channel in each direction isoccupied by a traffic stream, thus efficiently utilizing networkcapacity. In a particular embodiment, the protectable traffic streamsare higher priority traffic streams for which a customer has paid apremium, whereas the terminable traffic streams are lower prioritytraffic streams.

At decisional step 1008, it is determined whether has been aninterruption of a working path of a protectable traffic stream. Suchinterruption may comprise a line cut or other interruption that preventsthe protectable traffic stream from reaching all of its destinationnodes. If no interruption has occurred, the method returns to step 1000.If an interruption has occurred, then, at step 1010, the interruption isisolated. In a particular embodiment, the isolation of the interruptioncomprises opening the clockwise ring at the add/drop node clockwise ofthe interruption by opening the switch 62 in the clockwise transportelement, and opening the counterclockwise ring at the add/drop nodecounterclockwise of the interruption by opening the switch 62 in thecounterclockwise transport element.

Proceeding to step 1012, any existing, terminable traffic is terminatedalong the protection path. Terminable traffic may remain if thecorresponding working path is not interfered with by a fiber cut orother interruption. At step 1014, the hub node is reconfigured to allowthe protected traffic to proceed along the protection path. In aparticular embodiment, this may be accomplished by closing thepreviously-opened switches 210 corresponding to that wavelength in thehub node.

Proceeding to step 1016, a switch in the DCE of the origination ADN ofthe interrupted traffic switches the direction of the interruptedtraffic. At step 1018, the interrupted traffic is transmitted in theprotection path. At decisional step 1020, it is determined whether theinterruption has been repaired. If the interruption has not beenrepaired, the method returns to step 1018 and the interrupted trafficcontinues to be transmitted in the protection path. If the interruptionhas been repaired, the method proceeds to step 1022 wherein the networkis reverted to its pre-interruption state, and the method has reachedits end.

FIG. 24 is a block diagram illustrating a multi-subnet optical ringnetwork in accordance with another embodiment of the present invention.The network of FIG. 24 may provide for 1+1 protection of traffic inlong-haul networks. Network 1100 of FIG. 24 comprises rings 1114 and1116. In the illustrated embodiment, each ring comprises a pair ofoptical fibers operable to transmit traffic in counter-rotationaldirections. Alternatively, each ring may comprise a singlebi-directional optical fiber. In the illustrated embodiment, gatewaynodes 1102 comprise the boundaries of three subnets. As described ingreater detail in reference to FIG. 25, add/drop nodes 1104 may beoperable to passively add and drop traffic from each of the rings 1114and 1116 and to maintain flexible channel spacing. In the embodimentsdescribed below in reference to FIGS. 26 and 27, gateway nodes 1102 areoperable to add and drop traffic from rings 1114 and 1116, and furthercomprise mux/demux pairs to form the boundaries of a plurality ofsubnets. In the illustrated embodiment of FIG. 24, three subnets—subnet#1, subnet #2, and subnet #3, are shown.

FIG. 25 is a block diagram illustrating an add/drop node of the networkof FIG. 24 in accordance with one embodiment of the present invention.Referring to FIG. 25, add/drop node 1104 comprises subnode 1150 which isoperable to add traffic to and drop traffic from optical ring 1116, andsubnode 1152 which is operable to add traffic to and drop traffic fromoptical ring 1114. Subnodes 1150 and 1152 each comprise amplifiers 64and couplers 60 as described above in reference to FIG. 2. Couplers 1154may comprise 2×4 passive couplers. In contrast to node 12 of FIG. 2,ring switches are not included in node 1104.

In one embodiment, node 1104 may receive from a local client two trafficstreams comprising identical traffic. For example, in the illustratedembodiment, a single local traffic stream is split at the local clientby splitter 1168 into two streams—a first stream and a secondstream—before entering add/drop node 1104. The first stream istransmitted via transponder card 1164 to subnode 1150 to be added toring 1114 via a coupler 60. The second stream is transmitted viatransponder card 1166 to subnode 1152 to be added to ring 1116. In theillustrated embodiment, the first and second streams are added to rings1114 and 1116 in the same direction (counterclockwise). In this way, 1+1protection is provided for the two locally-added traffic streams in caseof a failure in one of rings 1114 or 1116 or in the associated addand/or drop devices within the network 1100.

Likewise, traffic streams from rings 1114 and 1116 may be dropped viacouplers 60 in subnodes 1150 and 1152 to filters 1156 and 1160.Locally-destined traffic may be selected by filters 1156 an 1160 andtransmitted to a local client via transponder cards 1158 and 1162. Inthis way, 1+1 protection is provided for the locally-dropped trafficstream in case of a failure in one of rings 1114 or 1116 or in theassociated add and/or drop devices within the network 1100.

FIG. 26 is a block diagram illustrating a gateway node of the network ofFIG. 24 in accordance with one embodiment of the present invention.Gateway node 1102 provides an optical-electrical-optical conversion oftraffic carried by FIG. 24, and allows for the adding of optical trafficfrom a local client or other sources.

Referring to FIG. 26, gateway node 1102 comprises subnodes 1200 and1202. Subnodes 1200 and 1202 each comprise amplifiers 64 as describedabove in reference to FIG. 2, a demultiplexer 1204, a multiplexer 1206.Subnode 1200 is operable to demultiplex traffic from optical ring 1116,to drop demultiplexed traffic, to add local traffic, and to multiplextraffic for forwarding on optical ring 1116. Subnode 1202 is operable todemultiplex traffic from optical ring 1114, to drop demultiplexedtraffic, to add local traffic, and to multiplex traffic for forwardingon optical ring 1114.

Demultiplexed traffic streams from rings 1114 and 1116 are forwarded toreceiver 1210 and a sender in transponder 1212. Locally-destined trafficmay be selected for dropping by optical switches 1214, and throughtraffic forwarded to multiplexers 1206. Optical switches 1214 may alsoadd locally-derived traffic and forward the locally-derived traffic tomultiplexers 1206. In this way, 1+1 protection is provided for thelocally-dropped traffic stream, for through traffic, and forlocally-added traffic in case of a failure in one of rings 1114 or 1116or in the associated add and/or drop devices within the network 1100.

FIG. 27 is a block diagram illustrating a gateway node of the network ofFIG. 24 in accordance with another embodiment of the present invention.The gateway node 1300 of FIG. 27 provides an optical-electrical-opticalconversion at the gateway forming a boundary between subnets, and may beused in place of gateway node 1102 described above in reference to FIG.26.

Referring to FIG. 27, gateway node 1300 comprises subnodes 1200 and 1202as described above in reference to FIG. 26. However, in contrast to node1102 of FIG. 26, electrical switches 1302 are operable to drop opticaltraffic converted to an electrical signal by receivers 1210. Electricalswitches 1302 or likewise operable to forward locally-derived trafficcomprising electrical signals to transponders 1212 to be converted to anoptical and to be multiplexed and added to optical rings 1114 and 1116via multiplexers 1206.

Although the present invention has been described with severalembodiments, various changes and modifications may be suggested to oneskilled in the art. It is intended that the present invention encompasssuch changes and modifications as fall within the scope of the appendedclaims.

1. A method of protection switching traffic carried on an optical ringnetwork, the ring network comprising a first optical ring, a secondoptical ring, a plurality of add/drop nodes and a hub node, the methodcomprising: detecting an interruption in a working path of trafficcarried on the optical ring network, the working path extending betweenan origination node and a destination node, the interruption preventingthe traffic from reaching the destination node from the origination nodevia the working path; isolating the interruption by opening the firstoptical ring at a first node adjacent in a first direction to theinterruption and opening the second optical ring at a second nodeadjacent in a second direction to the interruption; terminating along aprotection path terminable signals of the wavelength of the traffic inresponse to detecting the interruption, the protection path comprisingthe hub node and extending between the origination node and thedestination node; reconfiguring the hub node so as to pass through thetraffic in response to detecting the interruption; and without changingthe wavelength of the traffic, forwarding the traffic along theprotection path and through the hub node to the destination node.
 2. Themethod of claim 1, further comprising reverting the network to apre-interruption state in response to at least a repair of theinterruption.
 3. An optical network, comprising: a first optical ringand a second optical ring; a plurality of add/drop nodes coupled to theoptical rings; a hub node coupled to the optical rings; means fordetecting an interruption in a working path of traffic carried on theoptical ring network, the working path extending between an originationnode and a destination node, the interruption preventing the trafficfrom reaching the destination node from the origination node via theworking path; means for isolating the interruption comprising means foropening the first optical ring at a first node adjacent in a firstdirection to the interruption and means for opening the second opticalring at a second node adjacent in a second direction to theinterruption; means for terminating along a protection path terminablesignals of the wavelength of the traffic in response to detecting theinterruption, the protection path comprising the hub node and extendingbetween the origination node and the destination node; means forreconfiguring the hub node so as to pass through the traffic in responseto detecting the interruption; and means for without changing thewavelength of the traffic, forwarding the traffic along the protectionpath and through the hub node to the destination node.
 4. The opticalring network of claim 3, wherein each of the plurality of add/drop nodescomprises means to passively add and drop traffic from the opticalrings.
 5. The optical network of claim 3, further comprising means forreverting the network to a pre-interruption state in response to atleast a repair of the interruption.